The present invention relates to deoiler or breather assemblies, and more particularly for deoiler or breather assemblies for use with gas turbine engine gearboxes.
Gas turbine engines and other mechanical devices can include gearboxes and/or bearing assemblies that utilize an oil flow for cooling and lubricating purposes. It is often desired to avoid pressuring bearing compartments and gearboxes, but instead to vent such compartments and allow them to “breathe”. In such an arrangement, oil can become mixed with vented air, causing oil saturation in that air. It is further desired to reclaim oil present in the vented air. The presence of oil in vented air that leaves an engine is unsightly and aesthetically undesirable. In particular, for gas turbine engines used in commercial airline applications, the visible clouds of oil in exhaust streams may be unpleasant to customers or passengers who prefer such exhaust streams to appear transparent—even if such exhaust streams are harmless and within accepted operating parameters.
In a typical prior art deoiler/breather assembly (the terms “deoiler” and “breather” are used synonymously herein), a fluidic mixture of oil and air in a bearing or gearbox compartment is passed through a rotating separator that draws oil out of the mixture. The oil removed from the mixture can then be returned to a primary lubrication circuit for further use. Remaining air from the mixture can leave the rotating separator through a tube or shaft located along a central axis of rotation and can be exhausted from the engine (and its nacelle) to ambient air. Such prior art deoiler/breather assemblies are able to efficiently retain oil to avoid losing too much oil through the vented air, though some small amount of oil typically remains in the exhaust stream of the remaining air. In a typical gas turbine engine, air in the deoiler/breather assembly is at elevated temperatures generally in the range of approximately 121-177° C. (250-350° F.). At elevated temperatures, oil can exist as vapor (i.e., in a gaseous state). However, condensation of small, dispersed oil droplets can exist in vented exhaust streams under certain circumstances. In particular, when vented air containing oil vapor is cooled by adiabatic expansion (i.e., a decrease in pressure) or by mixing with colder air, the oil vapor can condense into tiny droplets (i.e., liquid state droplets) that can reflect light in the visible spectrum and appear as “white smoke”, that is, as a visible cloud of material that can appear to be smoke from a combustion process to an unfamiliar observer.
Prior art solutions to the problem of visible oil in exhaust streams from deoilers/breathers include dispersing such exhaust streams in a fan bypass stream from the engine, which combines the oil-containing exhaust stream with such a large volume of oil-free air that the oil is greatly dispersed and not readily visible. However, this solution requires that an exhaust port for the deoiler/breather to have a particular location in relation to the fan bypass air stream (typically an exhaust port near an aft end of the engine), which is not always feasible for certain engine and nacelle configurations. In the past, efforts have also been made to improve air/oil separation so that less oil is present in exhaust streams from a deoiler/breather. However, even with such efficiency improvements, the separation process is not 100% efficient and some small amount of oil will remain in exhaust streams that may become visible. In addition, some deoiler/breather assemblies have included a cruciform structure on an interior of a rotating exhaust shaft or tube to eliminate a “free” vortex that can lead to oil condensation in the exhaust stream by regulating vortex rotation with the cruciform structure. However, because such cruciform structures rotate with the shaft of the separator, they must be rotationally balanced, which is difficult to accomplish.
Thus, an improved deoiler/breather assembly is desired.